The fluids conveyed by these tubings can be of very different kinds, and most often these tubings will be used for oil extraction. It is often interesting to transport oil at its bottomhole temperature, thus avoiding a certain number of drawbacks generated by heat exchanges with the outside. These drawbacks are for example paraffin, asphaltene, hydrate or viscous crude deposits under surface conditions, condensation or instability phenomena, productivity losses. These drawbacks can generate phenomena referred to as self-killing phenomena and lead to premature well or even field abandonment.
A known solution consists of thermally insulating the production tubing so as to minimize heat losses as the fluids flow therethrough, with the aim of keeping the thermal conductivity of these insulated tubings in the 0.05-0.2 watt/m/.degree. C. range.
Various thermal insulation methods for tubings exist and have already been applied in production fields. A distinction can be made between complete tubing insulation processes using various fluids, for example gas, oils, aqueous or organic gels. molten sulfur, and processes for insulating the outer wall of the production tubing, for example lining using insulating materials, aluminium-based insulating paints, rigid foams and polysilicate foams generated in situ.
All these techniques are either expensive or not very effective, or hardly reversible. Using fluids whose reversibility would not be total furthermore presents risks of damage to certain producing formations as weakly soluble products are brought into contact. The reversibility of the systems used thus appears to be essential.